The present invention relates to a signal quantizer employing Josephson junctions and to a high resolution A/D converter, using the signal quantizer.
There are two major ways to build a high resolution N-bit A/D converter using Josephson junction circuits. The first way is to use a superfast binary counter to serially count the number of flux quantum transitions of a single Josephson junction in response to an input analog signal. The second way is to resolve all bits of the analog signal in parallel using a set of .PHI..sub.0 -periodic comparators based on two-junction superconductive quantum interferometer devices (SQUID), .PHI..sub.0 =2.07*10.sup.-15 Wb being the magnetic flux quantum.
The A/D converter of the first type has very high precision but it can convert only very low-frequency signals. The converter of the second type is operable at sampling rates exceeding 1 GHz, but it has a low resolution, because to get N correct bits it requires the parameter margins of the divider and comparator to be as small as 2.sup.-N.
Rylov et al.'s paper entitled "Josephson Junction A/D Converters using Differential Coding," IEEE Transactions on Magnetics, Vol. MAG-23, No. 2, pp. 735-738, March 1987, describes a high speed high resolution A/D converter based on a signal quantizer called "SQUID wheel." Rylov's SQUID wheel has 2.sup.p "spokes" consisting of identical Josephson junctions and small inductances, connected between a wheel rim and center node of the wheel, p being a positive integer, such that successive junctions are phased 2.pi./2.sup.p apart. The entire wheel is shunted by another inductor (not shown by Rylov) so that all of the junctions respond periodically to the inductor's external magnetic flux created by the analog input signal with the same built-in period .PHI..sub.0. The very high accuracy of the .PHI..sub.0 -periodicity of the SQUID wheel is determined by the existence of flux-quantization in the superconductors, one of the fundamental preservation laws of nature. This extremely high accuracy of the .PHI..sub.0 periodicity allows the automatic matching of the elements in the p-bit interpolator over a very large dynamic range and therefore can be used to achieve very high resolution of the A/D converter without sacrificing the speed of the A/D conversion.
In the Rylov's embodiment the center of the wheel is grounded and the analog signal is injected into the rim. However, a potentially large analog signal in Rylov's embodiment can create crosstalk in the presence of a finite inductance rim, which would disrupt the relative junction phases.
In FIG. 3, Rylov illustrates how to read the SQUID wheel with r (=2.sup.p-1) comparators. In an example given by Rylov, p=3, and r=4. However, the number of superconducting wiring crossovers in such a scheme is also r=4, which creates a severe problem, because in the most complicated superconductor IC processes, the number of superconducting wiring crossovers is 1-2. Furthermore, Rylov proposed read-out geometry is plugged with crosstalk and imbalance problems.
Thus, while the Rylov SQUID wheel is a step in the right direction for achieving a high resolution high speed A/D converter, it is nonetheless impractical. Therefore, it is desirable to provide a practical and improved high resolution high speed A/D converter.